Of molded articles, electric and electronic parts used in an OA (Office Automation machines)-field for some specific uses are required to be of resin compositions excelled in physical properties such as flame retardancy, impact strength and heat resistance. For example, required are those not less than 40 kg-cm/cm in notched Izod impact strength and heat resistance of not less than 100.degree. C. in HDT (heat distortion temperature). Besides those properties, chemical resistance and moldability are further required. As such resin compositions, polycarbonate-type resins or ABS resins are being used widely.
In recent years, especially with resins used as materials of electric or electronic parts, are required to be safe to fire, hence the material resins are often required to have a high degree of flame retardancy to pass UL-94 (U.S. Underwriters Laboratory Standard) V-0 and, therefore, various flame retardants have been developed and studied.
For the impartation of such a high degree of flame retardancy to a resin composition, generally used is a halogen-type compound as a flame retardant, if necessary, in combination with a flame retardant auxiliary such as antimony trioxide. Halogen-type compounds with their high flame retardant effect, however, have such defects as free halogen gases or halogen compounds resulting from the decomposition of the halogen-type compounds during resin processing is likely to cause corrosion of an extruder cylinder or die surface or corrosion of metal portions of electric or electronic parts, which possibly causes contact failure or poor conduction. The gases resulting from the decomposition of the halogen-type compounds include poisonous ones in extremely minute amounts, Hence, strongly desired is development of a flame retardant resin composition not containing even traces of halogen-type compounds.
As one of such flame retardants, a kind of phosphorus-type compounds is being studied in good earnest with regard to their application possibilities. Generally, however, the flame-retarding effect of the phosphorus-type compounds is lower than that of halogen-type ones, hence addition of a greater amount thereof is required if a high degree of flame retardancy is needed. And this results in lowering of physical properties such as impact strength and heat resistance and it comes to be extremely difficult to keep them sufficiently high together with flame retardancy.
Various studies have been made to date for solving such problems and resin compositions comprising polycarbonate resins, polyalkylene terephthalate resins, graft copolymers, phosphorus-type compounds and fluorocarbon-type resins with or without limitations of their blending ratios are disclosed in, for example, Japanese Laid-open Patent Publication No. 6-192553 and Japanese Patent Publication No. 6-242055.
The resin compositions mixed with such phosphorus-type flame retardants, unlike those mixed with halogen-type flame retardants, have particular phenomena caused by the decomposition of the phosphorus-type flame retardant during resin processing such as yellowing of molded articles and easy formation of silver streaks in the surface thereof. As to yellowing, in particular, there is a further problem, among others, of increased yellowing of the molded articles when they are used under high-temperature conditions.
Of the required physical properties, especially important is the chemical resistance, which is often required to be much higher with spread of uses for such electric parts in recent years.
An extensive series of studies have been made for solving of such problems and with regard to the improvement of resistance to heat-induced discoloration, flame retardant resin compositions admixed with specific organic phosphoric diesters and alkali metal salts thereof as additives for polycarbonate resins are disclosed in, for example, Japanese Laid-open Patent Publication No. 5-9371 and Japanese Laid-open Patent Publication No. 6-207088. Although this method is effective against discoloration of resin compositions dwelling in an injection molding machine, organic phosphoric esters and alkali metal salts thereof accelerate thermal decomposition of polycarbonate resin which enhances silver streaks formation. In Japanese Laid-open Patent Publication No. 6-228426, Japanese Laid-open Patent Publication No. 5-1079 and Japanese Laid-open Patent Publication No. 5-92986 there is disclosed a method for decreasing the degree of discoloration of molded articles by the use of specific phosphoric esters as a phosphorus-type flame retardant. The disclosed method, however, may not be sufficiently effective or, worse, even fail to improve chemical resistance or to prevent silver streaks formation.
For the simultaneous improvement of effect for increasing resistance to heat-induced discoloration and decreasing silver streaks formation, a method of adding organic phosphorus-type compounds and melamine cyanurates to polyamide resins is disclosed in Japanese Laid-open Patent Publication No. 6-145508. The melamine cyanurates, however, accelerate foaming of polycarbonate-type resin and cause increased dripping, hence this method is difficult to apply to polycarbonate-type resin compositions.
Meanwhile, as a method of improving the chemical resistance of polycarbonate resins, there is disclosed in, for example, Japanese Published Patent Publication No. 5-87540, a method of adding graft copolymers and high-density polyethylene resins to a resin composition of polycarbonate resins and thermoplastic polyester resins, but there arises a problem of the resin composition flame retardancy being deteriorated by the high-density polyethylene resins as it is applied to a flame retardant recipe. In Japanese Laid-open Patent Publication No. 5-21754, there is a description about a method of improving the chemical resistance of flame retardant polycarbonate resin compositions using halogenated bisphenol-type epoxy resins, but no reference is made to resistance to heat-induced discoloration or silver streaks formation, and the problems caused by halogen-type compounds are still involved.
Although, as mentioned above, there are proposed various methods for improving resistance to heat-induced discoloration as well as for prevention of silver streaks formation, there are still included problems of the intended improving effect being insufficient or of causing deterioration of the innate excellent physical properties of the resin compositions. Under such conditions, there have been awaited development of halogen-free resin compositions excelled in physical properties such as flame retardancy, impact strength, heat resistance and moldability and, more important, also excelled in chemical resistance and resistance to heat-induced discoloration and silver streaks formation.
The present invention is aimed at providing a halogen-free flame retardant resin composition excelled in physical properties such as flame retardancy, impact strength, heat resistance and moldability and, more important, also excelled in chemical resistance and resistance to heat-induced discoloration and silver streaks formation.
The present inventors made extensive and intensive studies for solving the aforementioned problems and to their astonishment discovered that a flame retardant resin composition satisfying the aforementioned properties was obtainable by blending in specific quantities a polycarbonate resin, a polyalkylene terephthalate resin in specific molecular weight, a specific copolymer, an organic phosphorus-type flame retardant and an epoxy compound not containing halogens, and thus could arrive at the present invention.